076 MR. W. CARRUTHERS ON FOSSIL CYCADEAN STEMS 



5 



structure (let nnines their systematic position as certainly as the seeds of the one and the 

 spores of the other. In the Cycadece the vascular cylinder is composed of disk-bearii 

 ti>sue, everywhere traversed by medullary rays; it invests a true cellular medulla, and 

 surrounded by a cellular cortical layer; while in Lepidodendron and its allies the vascul 



vlinder consists of scalariform tissue, without true medullary rays ; it encloses an axis of 



large irregular scalariform tissue, and is surrounded by a somewhat complicated outer 

 layer, composed, in the inner portion, of parenchyma, and in the outer, of prosenchyma 1 . 

 Unfortunately the minute structure is not preserved in Eichwald's stems. But there 

 appear to me to be, nevertheless, in Tessellaria antiqua and JDiplodendron hastatum the 

 means of determining with some certainty to which of the groups they belong. I refer 

 to the proportion which the vascular cylinder bears to the diameter of the stem. Iu the 

 paheozoic cryptogams the cellular cortical layer is enormously developed, and the vascular 

 cylinder is correspondingly reduced, while in the Cycadece the vascular cylinder is larger 

 in d n rarer to the circumference of the stem. The two fossils figured by Eichwald agree 

 in these particulars with Lepidodendron; and I accordingly prefer to place them in thai 

 genus, as was originally done by Kutorga. Goppert 2 has removed one of these stem- 

 {Diplodendron hastatum) from Cycadece and placed it among the Lepidodendrece ; but he 

 retains T. antiqua among the Cycadece, without, however, giving any reason for separating 

 two steins u hicli agree in all points of structure in which they can be compared. I have 

 been greatly assisted in the critical examination of these stems by the casts of them pre- 

 sent) I to the British Museum by Sir E. I. Murchison. The materials for estimating the 

 affinities of the other five species described by Eichwald are less satisfactory. There are 

 no characters by which Zamites microlepis and Tessellaria squamosa, retained by Goppert 

 in Cpcadea, can be separated from lepidodendron. T. biarmica, considered by Goppert 

 to be a fern-stem, is a "decorticated" specimen of a plant of the same genus; and 

 Zamites drigaius and Z. densifolius, referred by Goppert to Cycadece, are too imperfect 

 and obscure to warrant even a guess as to their affinities. 



The structure of Medullosa, Cotta 3 , and Colpoaylon, Brongn. 4 , referred by Goppert, 

 Miquel, and Unger to Cycadecv, differs so essentially from any known stem of that order, 

 that I would even go further than Brongniart, who considers their Cycadean claims to 1>. 

 very donbttul, and say that, as far as the evidence supplied by the stems goes, their affini- 

 ><- are much more with the vascular cryptogams than with any tribe of phanerogams. 



-v> satisfactory evidence, then, exists, as far as I am aware, of the occurrence of Cycadeo 

 n. any i aheozo.e formation. In strata of Secondary age their remains are both abundant 

 ana characteristic. Stems, leaves, and fruit are so perfectly preserved that their true 

 name can b determined with the utmost precision. They are, on the other hand, 



usably aUent from beds of Tertiary age. The stem from the Paris basin, referred 

 5 n-esl to tins order, under the name Zamltes JBrongniarti^ is, as Brongniart originally 



£> 



£iiiart 



&er - I « the Structure of the Stems It W I' ^ T*** ****"* * QyC "' IAnm > 1BU > P * ** ^ 

 \ n „,» ,.._,-, ems ot the Arborescent LvcoDodiacfis* nf rt,* nn-1^—— » ai^+vw Afieroscop. 



Joan. vol. i. p. I-;. 



lycopodiacese of the Coal-measures." Monthly Microscop. 



& _ 



1 Ke I lithea in i!^^^ < -i ' Fossile Flora der Pcrm - Form - P- 132. 



r.ihl. Qea, Y.- Fos S . p> ^ 



* Sternberg, Flora d. Vorwelt, ii. p. 196. 



